在python中遞歸地添加行到字符串

Question

我想添加命令來繪製圖形到字符串。因此，我寫了一個遞歸函數。

def draw(root,string): 
    string += 'Commands to draw the root' 
    if root.left != None: 
     string += 'Commands to draw a left child' 
     draw(root.left,string) 

    if root.right != None:...#same as for the left child 

我很困惑。如果我使用的功能就是這樣，我的字符串並沒有改變：

>>>a = '' 
>>>draw(root,a) 
>>>print(a) 
>>>a 
'' 

我嘗試添加「返回字符串」，但在這種情況下，我的功能與繪畫的根源，其左，右完成後停止兒童。

作爲一個例子：

• 根= 3
• root.left = 2
• root.right = 5

• 2.left = 1

  a='' draw(3,a) a

預期輸出：

'Commands to draw 3, Commands to draw 2, Commands to draw 5, Commands to draw 1'

Answer 1

你string arg是該函數的局部變量，因此除非您返回，否則您對該函數進行的任何重新分配都是該函數的局部變量。 Python字符串是不可變的，所以你實際上不能修改傳入的字符串，你所能做的就是重新分配給它，從而創建一個新的本地字符串對象。

所以不是將字符串作爲參數傳遞，而是從遞歸調用中返回。

我認爲這段代碼是做你想做的。我創建了一個非常簡單的Node類來測試它。

class Node: 
    def __init__(self, data): 
     self.data = data 
     self.left = None 
     self.right = None 

def draw(node, depth=0): 
    tab = 4 * ' ' * depth 
    depth += 1 
    string = tab + 'Commands to draw node. ' + node.data +'\n' 
    if node.left is not None: 
     string += tab + 'Commands to draw a left child.\n' 
     string += draw(node.left, depth) 
    if node.right is not None: 
     string += tab + 'Commands to draw a right child.\n' 
     string += draw(node.right, depth) 
    return string 

tree = Node('A') 
tree.left = Node('B') 
tree.right = Node('C') 
tree.left.left = Node('D') 
tree.left.right = Node('E') 

s = draw(tree) 
print(s) 

輸出

Commands to draw node. A 
Commands to draw a left child. 
    Commands to draw node. B 
    Commands to draw a left child. 
     Commands to draw node. D 
    Commands to draw a right child. 
     Commands to draw node. E 
Commands to draw a right child. 
    Commands to draw node. C 

Answer 2

你需要返回它在funtion的局部範圍otherwhise您的修改入住..

def draw(root,string): 
    string += 'Commands to draw the root' 
    if root.left != None: 
     string += 'Commands to draw a left child' 
     draw(root.left,string) 

    if root.right != None:...#same as for the left child 
    return string 

然後

>>>a = '' 
>>>a = draw(root,a) 
>>>print(a)